There is well known a vehicle provided with a step-variable transmission disposed between a drive power source and drive wheels. JP-2006-9942A discloses an example of this type of vehicle. This vehicle is a hybrid vehicle provided with an engine, planetary gear sets functioning as a differential mechanism, and first and second electric motors operatively connected to rotary elements of the planetary gear sets in a power transmittable manner. JP-2006-9942A also discloses a technique for calculating output torques of the first and second electric motors on the basis of their electric current values, estimating an input torque of the step-variable transmission on the basis of the calculated output torques of the first and second electric motors, and controlling transient hydraulic pressures to be applied to coupling devices of the step-variable transmission to implement its shifting actions, on the basis of the estimated input torque.
By the way, the known step-variable transmission may include a one-way clutch, which is placed in its engaged state to establish a predetermined one (first speed gear position, for example) of gear positions of the step-variable transmission. In the hybrid vehicle as disclosed in JP-2006-9942A, a regenerative control of an electric motor is implemented when a reverse or negative drive torque is transmitted in a direction from the drive wheels toward the step-variable transmission during running of the hybrid vehicle while the step-variable transmission is placed in the above-indicated predetermined gear position. When the negative drive torque is transmitted to the step-variable transmission, the one-way clutch is brought into its released state, and the predetermined gear position of the step-variable transmission is not established, so that it is difficult to implement the regenerative control of the electric motor. To prevent this difficulty, the step-variable transmission is provided with a coupling device disposed parallel to the one-way clutch. The coupling device is placed in an engaged state during running of the vehicle with the step-variable transmission being placed in the predetermined gear position, so that the step-variable transmission is held in the predetermined gear position even when the negative drive torque is transmitted to the step-variable transmission, whereby the regenerative control of the electric motor can be implemented.
When the step-variable transmission is shifted up from the predetermined gear position to another gear position, the coupling device disposed parallel with the one-way clutch is brought into its released state. In the event of generation of fluctuation such as resonance in the step-variable transmission during this shift-up action of the step-variable transmission, an excessively large torque acts on the one-way clutch, giving rise to a risk of reduction of durability of the one-way clutch. If a releasing action of the coupling device is delayed to prevent this risk, an engaging hydraulic pressure of the coupling device becomes excessively high with respect to an input torque of the step-variable transmission which is rapidly reduced as a result of a releasing action of an accelerator pedal in the process of the shift-up action. In this event, the step-variable transmission is placed in a tie-up state, giving rise to a risk of generation of a shifting shock upon initiation of an inertia phase of the shift-up action.